The present circuit topology is based on an already known MOSFET circuit topology for bidirectional signal interconnection that is also referred to as “common source” and that is illustrated in FIG. 1.
In order to switch the transistors to a conductive or “on” state, a control current is necessary which generates the required gate-source voltage across the resistor R1 and charges the input capacitances of the two gates T1 and T2. If this control voltage is galvanically coupled to the signal voltage, it must be greater or less than the signal voltage depending on MOSFET types used (N- or P-channel), within the scope of the transistor specification, in order to switch T1 and T2 to the on state. This may be technically difficult in the case of high-voltage signals. What is also disadvantageous is that the control current can be superposed with the signal current and flows via the terminals A or B to the opposite potential. As a result, this circuit variant is unfavorable for an interconnection of measurement voltage signals.
Therefore, the control current is often generated photovoltaically (see FIG. 2), which firstly provides for the galvanic isolation for the purpose of driving, and secondly prevents the control current from being superposed with the signal current. What is disadvantageous here is that the driving of the gates of T1 and T2 requires a relatively high control current which must primarily first of all supply the light emitting diode LED1. The control current generated on the secondary side, for example, from a photo diode D1, is relatively small, which is an obstacle to rapidly switching on T1 and T2.
If this type of switch is required multiple times, for example in an implementation of a multiplexer, this type of galvanically isolated driving has to be realized individually for each transistor pair. This is technically complex.
In test engineering and metrology, for example for automated connection testers, signal voltages and currents need to be connected by a switching matrix which consists of up to several thousand individual switches and has to interconnect currents that are well up to the amperes range and voltages of up to a few kilovolts. If the signal voltage is a dangerous contact voltage, for safety reasons, the requirements for air clearances, creepage paths, and insulation from the respectively relevant safety standard (for example, IEC60950, IEC61010 et cetera) must be complied with for each driving, in order to reliably isolate driving and switches from one another. The large distances, owing to the mixed construction of switching element, on the one hand, and drive lines, on the other hand, which must also be insulated from the other switching elements of the matrix, make it more difficult to implement the construction with a high packing density, which makes the construction more expensive, or causes the construction to become larger.
A further alternative for galvanic isolation in relation to the photovoltaic driving is capacitive driving, by two small capacitors. However, the latter can only transmit AC signals, which then ultimately have to be rectified again in order to supply the required gate-source voltage for T1 and T2.
Furthermore, the complex, expensive alternative of driving by a transformer is known. Here as well, only AC signals can be transmitted, which then have to be rectified on the secondary side.
The galvanic isolation during driving prevents the drive current from being superposed with the current to be interconnected and allows the drive voltage to be able to have a different potential than the signal voltage. A technical difficulty is posed here by the energy to be transmitted, which has to be transmitted as efficiently as possible via the galvanic isolation in order to charge the gates of the two transistors T1 and T2, in order that the latter can switch into the on state rapidly enough. In conventional driving by photovoltaics, a relatively large amount of energy is required in order to compensate for the losses of the LED-receiver diode coupling section, which would lead to high power losses in the case of a multiplexer arrangement having many switches.